The present invention relates to a control system for a continuously variable belt-drive automatic transmission (CVT) for a motor vehicle, and more particularly to a system for controlling the transmission ratio changing speed rate in accordance with the transmission ratio and the engine speed.
In such a system, in order to prevent overshooting or hunting at a transient state, the convergence of the transmission ratio must be improved as well as the response of the system. A transmission ratio changing speed control system has been heretofore proposed, in which the changing speed rate of the transmission ratio is controlled in accordance with the difference between the actual transmission ratio and a desired transmission ratio. The desired transmission ratio is corrected to obtain an optimum value in every driving condition, thereby improving the feeling of acceleration.
As shown in FIG. 5a, at acceleration of a vehicle, the desired transmission ratio id is quickly increased in accordance with the depression of the accelerator pedal of the vehicle. Thereafter, as the vehicle speed increases, the desired transmission ratio id gradually reduces. The transmission ratio changing speed rate is determined to control the actual transmission ratio i it converges toward the desired transmission ratio id. At that time, acceleration G of the vehicle occurs as shown in FIG. 5c, which remarkably affects the feeling of acceleration. That is, at acceleration, a driver feels a maximum acceleration Gm, a time t to reach the maximum acceleration Gm from a time of starting of the acceleration, and the difference between the feeling of acceleration and the driver's requirement.
In low speed driving, a large maximum acceleration Gm which quickly increases within a short time is preferable to satisfy the driver's desire for a large acceleration.
In high speed the driving, drive pulley speed Np and driven pulley speed Ns (vehicle speed) of the CVT become high even if the actual transmission ratio i is the same as that at low speed driving. Accordingly, an increment .DELTA. Np of the drive pulley speed becomes large with respect to the same transmission ratio changing speed rate di/dt as the low speed driving. If the transmission ratio is quickly increased to a large value, engine speed quickly increases, which gives an unpleasant feeling like engine racing to the driver. Accordingly, it is preferable to keep the increment .DELTA. Np of the drive pulley speed Np constant regardless of the magnitude of the acceleration and the vehicle speed.
Japanese Patent Application Laid-open 59-208253 discloses a system for controlling the transmission ratio at kickdown. The transmission ratio changing speed rate changes gently at an early stage of acceleration in low speed driving or at low load of the engine, and increases quickly after a predetermined time. However, since the transmission ratio changing speed rate is reduced at the early period of acceleration, the transmission does not produce sufficient driving torque. Therefore, the system does not meet the requirement for rapid acceleration as obtained at kickdown.